The present invention relates to a new and improved method, of, and circuit arrangement, for testing an ultrasonic alarm installation.
In its more particular aspects, the present invention specifically relates to a method of testing an ultrasonic alarm installation in which ultrasonic waves of a predetermined frequency are emitted by an ultrasonic transmitter into a monitored region or space. Ultrasonic waves from the monitored region or space are received by an ultrasonic receiver. The received ultrasonic waves are converted into an electrical signal which is fed to an electrical evaluation circuit. This electrical evaluation circuit triggers an alarm signal upon a predetermined frequency deviation of the received from the emitted ultrasonic waves. A frequency deviation is also generated during a test phase and simulates an alarm state.
Such a method is known, for example, for Swiss Pat. No. 612,026. In such method the fact is utilized that a person moving in the monitored region causes, due to the Doppler effect, a frequency shift in a part of the ultrasonic waves by an amount corresponding to the speed of movement. This frequency shift is signaled by the evaluation circuit as an alarm state. To this end the frequency of the received ultrasonic waves is continuously compared with the transmitting frequency, and an alarm signal is triggered at frequency deviations characteristic for a moving person as described, for example, in Swiss Pat. No. 556,070.
In the method described in Swiss Pat. No. 612,026, a function control of the ultrasonic alarm installation is achieved due to the fact that, during a test phase, the pure transmission frequency is not utilized as the comparison frequency for the receiver, but a modulated oscillation. The modulation is selected such that after mixing with the receiver signal there is formed a Doppler signal which corresponds to that of a moving person. Also the propagation time difference between the ultrasonic waves which travel through the monitored space and an electrical reference signal which is transmitted directly from the transmitter to the receiver or the evaluation circuit, can be used for function control.
These known ultrasonic alarm installations in which ultrasound of a predetermined frequency is continuously emitted into the monitored region or space, however, have the disadvantage that a standing wave field is formed in the monitored region or space. In this standing wave field the ultrasonic waves reflected at various points within the monitored region or space are vectorially added to the emitted ultrasonic waves in the receiver to form the receiver signal in dependence upon their amplitude and phase relationship. Depending on the arrangement of the reflecting objects in the monitored region or space and the environmental conditions, the receiver signal can therefore fluctuate within wide limits from zero to a maximum. Additionally the signal continuously changes within wide limits, for example, due to temperature fluctuations, changes in the humidity of the air or the air pressure, and accordingly the sensitivity of the alarm installation fluctuates uncontrollably. In the presence of a weak receiver signal the multiplication with a modulated reference signal, however, produces only a weak Doppler signal so that the function control becomes ineffective. Therefore, also changes in the ultrasonic field, e.g. by installation of a sound absorbing wall in front of the transmitter during a sabotage attempt, cannot be distinguished from accidental fluctuations of the wave field.